The present disclosure relates to nitride semiconductor light-emitting elements, particularly to nitride semiconductor light-emitting elements including a protective film provided on a light-emitting facet.
Among semiconductor light-emitting devices, semiconductor laser devices have widely been used as a light source for reproducing and recording data of optical disc media such as compact discs (CD), digital versatile discs (DVD), Blu-ray™ discs, etc. In particular, the semiconductor laser devices have been required to record information at high speed, and higher output has been indispensable for the semiconductor laser devices to record more information in a short time. In recent years, high output semiconductor laser devices, which are inexpensive and stable solid light sources, have been required as light sources for displays, such as projectors, and light sources used for processing, such as laser annealing.
In general, when an optical output of the semiconductor laser device increases, the semiconductor laser device is deteriorated, e.g., an operating current gradually increases, or optical breakdown in which the semiconductor laser device stops oscillation occurs. Thus, researches and developments have been conducted to reduce or prevent the deterioration of the semiconductor laser device for higher output of the semiconductor laser device. In particular, a sudden death failure accompanied by the optical breakdown occurs at a front facet which is a light-emitting surface of a cavity constituting a semiconductor laser element, and efforts have been made to strengthen and stabilize a protective film covering the facet of the cavity. The facet of the cavity of the semiconductor laser element is usually covered with the protective film. The protective film controls a reflectance at the facet of the cavity, prevents adhesion of foreign matters on the facet, and prevents oxidation of the facet. FIG. 13 shows a schematic cross-section of a conventional nitride semiconductor laser element disclosed by Japanese Unexamined Patent Publication No. 2007-318088. As shown in FIG. 13, the conventional nitride semiconductor laser element includes a laser structure 400 including an active layer (a light-emitting layer), and an n-type semiconductor layer and a p-type semiconductor layer sandwiching the active layer. Although not shown, a p-side electrode is formed on the p-type semiconductor layer, and an n-side electrode is formed on the n-type semiconductor layer.
A protective film 406, which is a multilayer film of metal oxide such as silicon oxide (SiO2), aluminum oxide (Al2O3), etc., is formed on a rear facet which functions as a mirror of a cavity of the laser structure 400. A first protective film 407 made of aluminum nitride (AlN) or aluminum oxynitride (AlON) is formed on a front facet of the cavity, and a second protective film 408 made of aluminum oxide (Al2O3) is formed on the first protective film 407.
A possible cause of the deterioration of the semiconductor laser element associated with the higher output is a solid phase reaction caused between the first protective film 407 etc. and facets of the semiconductor layers due to generation of heat and absorption of light by the laser structure 400. Another possible cause is an interface reaction at the facet of the cavity in which residual oxygen in each protective film or oxygen in a package is diffused in the protective films as laser oscillation occurs to oxidize the facet of the cavity.
As heat is generated in the laser oscillation, physical breakdown occurs, e.g., the protective films may peel off the laser structure 400, or the laser structure 400 may be cracked. An AlN film used as the first protective film 407 shown in FIG. 13, or an AlON film in which a composition ratio of oxygen is 0.2 or lower is a thin crystalline film, and is an excellent material as the protective film on the facet for its strength and high thermal conductivity. In particular, AlN is a nitride like the nitride semiconductor, and the solid phase reaction with the laser structure 400 can be reduced. However, it has been known that these materials show a light absorption band near 400 nm derived from crystal defects etc., and heat generation, diffusion of oxygen, and oxidation may be caused by the absorption of light in the laser oscillation.
Japanese Unexamined Patent Publication No. 2008-147363 discloses a protective film made of AlN to which a rare earth element is added to reduce the diffusion of oxygen. Addition of the rare earth element to AlN or AlON can reduce the diffusion of oxygen because the rare earth element and oxygen are bonded. With use of the AlN film added with the rare earth element, the oxidation of the facet of the cavity and peeling of the protective films can be prevented, the deterioration of the facet and the optical breakdown can be reduced, and therefore, laser operation can be performed for a long time as compared with the case where a normal AlN film is used.